Evaluation of excavation effects on rock-mass permeability around the waste-handling shaft at the WIPP [Waste Isolation Pilot Plant] site

Pulse-injection permeability tests were performed in six subhorizontal boreholes at four horizons in the Waste-Handling Shaft at the Waste Isolation Pilot Plant (WIPP) site to evaluate potential changes in permeability caused by stress relief around the shaft. The test were performed using a triple-packer test tool capable of simultaneous isolation and pressure and temperature monitoring of three intervals within a borehole. The tests were designed to provide a ''profile'' of permeability with increasing distance from the shaft wall. All lithologies tested had very low hydraulic conductivities, with values ranging from 6 /times/ 10/sup /minus/15/ to 1 /times/ 10/sup /minus/13/ ms. No significant changes in permeability as a function of distance from the shaft wall were observed. Thus, if a zone of enhanced permeability caused by stress relief exists around the shaft, it is probably within the first 1 to 2 m of rock around the shaft that could not be tested using the available test-tool configuration. The testing revealed the presence of a depressurization zone around the shaft, apparently caused by drainage of fluid from the formation into the shaft. 5 refs., 5 figs